An image displayed on a cathode ray tube (CRT) may suffer from imperfections or distortions such as defocusing that is incident to the scanning of the beam on the CRT. Such imperfections or distortions occur because the distance from the electron gun of the CRT to the faceplate varies markedly as the beam is deflected, for example, in the horizontal direction. Reducing the defocusing that occurs as the beam is deflected in the horizontal direction, for example, may be obtained by developing a dynamic focus voltage having a parabolic voltage component at the horizontal rate and applying the dynamic focus voltage to a focus electrode of the CRT for dynamically varying the focus voltage. It is known to derive the parabolic voltage component at the horizontal rate from an S-correction voltage developed in an S-shaping capacitor of a horizontal deflection output stage.
The CRT that employs dynamic focus may have internal wiring that places the dynamic focus voltage close to, for example, the blue electron gun. In normal operation, the proximity to the blue electron gun may not cause any problem. However, when a low current bias measurement is made in an automatic kine bias (AKB) circuit, during several video line times that immediately follow vertical retrace, referred to as the AKB measurement interval, stray coupling of the horizontal component of the dynamic focus voltage may introduce an error in the biasing of the cathode electrode of the blue electron gun. As a result, the bias of the blue electron gun may not track the bias of the green and red electron guns. This may lead to unacceptable background color temperature changes.
It may be desirable to remove the horizontal dynamic focus voltage component from the focus electrode during the AKB measurement interval. Thereby, the undesirable coupling to the focus electrode is, advantageously, eliminated. During the AKB measurement interval, the value of the focus voltage may drift, due to the removal of the dynamic focus voltage component. After the end of the AKB measurement interval, a significant transient of the focus voltage may occur when the focus voltage is returned to its proper value.
It may be desirable to address a problem of picture distortion that occurs when the dynamic focus high voltage amplifier is powered with a deflection-derived power supply, especially when the horizontal frequency is a multiple of normal broadcast horizontal frequency. The dynamic focus signal is amplified to very high peak to peak voltages. The amplifier that does this job uses more power when operated at a high frequency, such as horizontal deflection frequencies in excess of 30 kHz, such as those used for multimedia and HDTV than it does for relatively low frequency 15 kHz used in conventional TV. When the high frequency variations cease during vertical blanking, the current drawn by the amplifier is greatly reduced. This causes a loading transient at the power source. If the power source is also providing scan deflection for the picture tube, as is often convenient, this loading transient may cause a distortion in the picture, such as width modulation ringing, that appears as wiggles in vertical lines in the top of the picture.